She was at the Moon Palace Hotel in Cancun, Mexico with her husband and their two children. She left Monday to go shopping but never returned.
Carla Burgos, her sister, said Mexican police have finally started to investigate.
"We're really lost. It's like a really big mystery," Burgos told CBS affiliate KCAL.
Bruce Beresford-Redman spoke to KCAL by phone Wednesday, but couldn't say much because he was hoping to keep the phone lines clear.
"I'm very hopeful someone will find my wife very soon. She still hasn't been located and we're looking," he told the station.
The Los Angeles County Sheriff's Department confirms they have a missing person's report and have notified the U.S. State Department and Embassy in Cancun.
In the meantime, family members said they are pleading for the public's help in this case asking anyone with information to come forward.
Monica Beresford-Redman was set to return Thursday, which would have been her birthday.